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Dunno if these observations will help anyone, but I noticed that as a spirit, attempting to get force/magic/edge as high as possible, you end up having to build all of your attributes up to 4 (by the design of the houserule), which makes you much more like a force-4 spirit than a force-9 spirit. Compared to the stock spirits in the BBB, your manifested form is actually a bit worse than a force-4, because their manifested physical attributes are usually a few points higher than their force. You could easily have most of the spirit powers you might want, of course, but if you have to throw down in astral combat, you're basically a force-4 spirit. Some of your powers will key off of your ridiculously high force/magic/edge, but for a lot of rolls you'll only be chucking 5-9 dice. I also noticed you can't actually become force-10, because raising all of your other attributes to 5 would cost more than half of your points (and you can't put more than half into attributes). Not that that is a huge deal-breaker, but it seemed strange that a non-spirit is welcome to have magic 10 and everything else a 2, but a spirit isn't allowed to do the same thing. My conclusion was that after dumping almost all of the BP into raising magic/edge/attributes, you end up as basically a force-4 spirit, who will probably be force-2 in the past.
Magic/force is not restricted by the other attributes (as was pointed out by someone else later), so you can start at 10 easy. If you start at 10, and are in the past in a background count of -7, and you have the funky spirit power I mentioned earlier, you will have force 4. A force 4 spirit with immunity to normal weapons 8 and the ability to cast force 4 spells will be extremely potent in the dark ages.
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Another thing I noticed when considering a mage, is that although you could easily have Magic 10, you wouldn't be unable to resist much more drain than usual. (Although being a grade 4 initiate with centering obviously helps). You might be able to spare the bonus points to max out your two resisting attributes, and pick focused concentration 2, but you're still talking about 16 dice (so 4-5 hits on average). So although you could cast force 10 spells, or summon force-10 great form spirits, and would have a lot of dice for the actual spellcasting roll (so you'd generate a lot of hits, and do something impressive), you'd probably knock yourself out with a couple of spells unless you reigned the force of your spells back down to something much more mediocre (like force 5). You still end up a good deal awesomer than a normal mage (getting 5 hits when a starting PC might get 3), but you are forced to act at a fraction of your true potential power.
With centering initiation grade adds to drain die. As an elf (which this game is designed around) you can start with 7 charisma, 5 willpower easily, 4 grades of initiation gives 16 soak die. Remember that as initiates you can get metamatic foci (including centering foci... admittedly only force 1 or 2) which can add a little more to your soak. Using bioware / genetics / martial arts you can get another 4 points. If you were minimaxing this gives around 22 soak die. Even if you don't minimax you can easily start with 16.
With only 16 die, using the 4:1 rule, you can throw force 11 stunbolts or force 7 stunballs all day/every day. With soak 20+ you are throwing force 13 stunbolts, force 9 stunballs.
As far as spell casting goes, with magic 9, spell casting 6, foci 2..4, mentor spirit and specialisation, you are throwing around 15..23 die in your spell casting.
Don't forget that once you have taken stun you can use first aid to soak it.
I'll point out the design of the soak mechanism is a shadowrun core rule, which I think is pretty reasonable.